Many mobile phones use a light emitting diode (LED) flash to improve image quality in low light conditions. The output power of these LED's has previously been limited by the amount of current that could be taken from the battery without tripping it's short circuit protection or exceeding the component's thermal limits, especially in multiple image capture situations (e.g., video or fast sequential image capture).
However, recent advances in LED technology and the commercial introduction of ‘super capacitors’ means that it is now feasible to pass high currents through a LED flash without tripping the battery and producing light intensities approaching those of a Xenon flash. Examples of such supercapacitors include the G and GZ series supercapacitors in the BriteFlash™ architecture by Cap-XX, Inc. of Sydney, Australia. Supercapactirors are also known as electrochemical mdouble layer capacitors (EDLC). Unfortunately, a mobile phone's internal temperature range varies commonly between 0° C. and 55° C., which severly limits the performance of practical implementations of such high-current embodiments to lower power levels so as to operate the LED safely at the higher end of that temperature range.
For the case where cameras have employed a pre-flash function (e.g., operating the flash immeidately prior to exposing the imaging surface/charge coupled display CCD during which time the flash is operated again to illuminate the scene being captured, commonly known as a red-eye reduction mode), the flash intensity has generally been kept the same for both the pre-flash and the full flash, and in both instances was kept within conservative thermal limits via designing the camera system operation for high ambient temperature.
It is also known that LED temperature can be measured or predicted based on its mathematical relation to forward voltage. From the temperature, thermal resistance of the LED junction can be calculated so as to predict a time delayed temperature at the LED once the voltage is removed, which of course may be accelerated by heat sinking the LED. Relevant and more detailed teachings in this regard may be seen at a paper by Jeff Hulelt and Chris Kelly entitled “Measuring LED Junction Temperature” (July 2008, http://www.photonics.com//content/spectra/2008/July/LED/92549.aspx, last visited Sep. 5, 2008).
Now arises a problem in that the confined spaces of a mobile phone or other handheld apparatus having an imaging function with a flash lead to the higher internal temperatures as noted above, which is where the imaging components lie. That same space constraint largely prevents increasing the heat sinking capacity about the LED since that would require thermal volume or a moving air mass. Those problems converge in the higher current LED implementations noted above. The higher LED forward current leads to higher heat outputs and thus a higher ambient temperature within the imaging apparatus, but there is no space available there to increase heat sinking via either volume or airflow.
It is well known that in low ambient light conditions a brighter flash is needed to adequately illuminate a scene for imaging, whether film or digital. But the super capacitor embodiments noted above rely on higher current, and so result in higher LED temperatures when flashing, and longer cooling periods absent improved heat sinking. Operating the flash at a lower than maximum intensity to account for design thermal constraints results in a degraded image quality because the deeper shadows in the scene are not sufficiently illuminated. Allowing a sufficient time for the LED to cool so as to operate it safely at maximum intensity would lead to a delay between the pre-flash and the main flash which in some instances, particularly in a mobile device with the Xenon-like higher intensity LEDs as above, may be so extended as to defeat the red-eye reduction purpose of the pre-flash. Additionally, consumers become readily frustrated by an extended delay between instances of sequential imaging because they may sometimes fail to capture an image they desire, such as when the subject in a scene is moving or changing facial expression.
These teachings are directed to addressing the above referenced problems.